The poor correlation between waviness and actual vibration measurements of assembled bearings at least for higher frequencies is a significant problem, as the vibrations cannot always be predicted after a waviness measurement using known technology. Also, known technology for effecting such a waviness measurement makes the measurement process rather time consuming.
Before now, it has only been possible to determine amplitude variations on the order of one hundred to ten nanometers in the production environment. However, a need exists for a method and apparatus that are able to measure amplitudes of a much smaller size. Such amplitudes might be on the order of or a part of the diameter of an atom or smaller.
The measurement is in particular needed for high precision bearings used in high precision instruments, bearings used in high speed applications such as household equipment and computer hard disc motors, and other applications to reduce noise caused by vibrations at a high frequency. Thus, the need for measuring apparatus and methods along the lines mentioned above is particularly acute in bearings where the bearing noise can be particularly disturbing because it occurs as a high frequency noise and in noise and vibration sensitive applications.
In the past, a lightly loaded pick-up has been used for measuring the waviness of different parts of bearings, such as balls, rollers, and rings. The pick-up is run in one or more narrow pathways on the surface to be tested. Variations in the bearing material, such as inhomogenities and surface roughness, influence the efficient waviness of the bearing, but is typically not correctly registered by the pick-up. The pick-up method thus does not measure all relevant effects which occur in real applications. The surface structure affects the pick-up in quite another way than a higher loaded rolling or rotating body working on a surface.
The difference might be due to the fact that only a fraction of the raceway area is measured, that local variations of the e-module in the subsurface material appear and affect the measurement; and/or that the EHD-film (ElastoHydroDynamic-film) thickness varies due to surface effects.
The rolling of a ball on a ring in a bearing affects the material down to, for example, a depth of thirty to forty microns on a small bearing, while a pick-up does not have such an impact on the material. This changes the requisites for a correct measurement of the functional behavior of the bearing. Additionally, a ball on a bearing ring is affected by the whole contact area and so by measuring just a fraction of the area it is thus not normally representative for the integrated value of the whole area.
It is of importance to measure the surface waviness of a bearing and its parts to obtain a tool or mechanism useful in quality determination after assembling such parts, i.e., so that the vibrations of the bearing can be predicted. It is of further importance to be able to control the process quality and thereby control the process equipment with regard to wear and other process parameters.
SE-C-314 214 discloses a method for testing the rotary movement of balls using a testing roller to check for the quality of the surface layer, the surface, or the shape. The roller is brought to act upon a ball with its surface being changeably active due to its shape. This document is not disclosed in the context of measuring waviness, but larger irregularities or unevenesses on the surface. The disclosed method is particularly adapted for use in connection with large balls.
In light of the foregoing, a need exists for a way of measuring the waviness of surfaces with higher accuracy than previously possible.
A need also exists for measuring the waviness on a larger surface (e.g., an adequate surface when considering a bearing surface) than with previously known apparatus to provide a functionality related integration, thereby making it possible to measure and integrate the influence of surface structure and surface roughness.
A need also exists to be able to produce bearings of higher quality, where the vibrations of a complete bearing can be predicted in an accurate manner.
A further need exists for controlling the process conditions under which the bearing parts are manufactured, as condition variations in the process will be monitored and considered.